Aircraft Structures: Wood and Fabric

Aircraft Structures: Wood and Fabric

Why Wood and Fabric Still Matter

Wood and fabric construction dominated aviation from the Wright brothers through the mid-twentieth century, and thousands of certificated aircraft still use these materials today. Gliders, homebuilts, and classic light aircraft such as the Piper Cub family rely on spruce spars, plywood ribs, and heat-shrink fabric covering. As an aviation maintenance technician, you are responsible for inspecting, repairing, and re-covering these structures to the standards in AC 43.13-1B, Chapters 1 through 3 (wood) and Chapters 4 and 5 (fabric). That advisory circular is your primary FAA-accepted reference throughout this lab.

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Wood Inspection Fundamentals

Aircraft-grade wood—primarily Sitka spruce—is selected for its high strength-to-weight ratio and straight, close grain. During inspection, you are looking for defects that reduce structural integrity:

• Moisture damage: Discoloration, soft spots, and fungal staining indicate rot. Probe suspected areas gently with an awl; soft penetration is a rejection criterion.
• Delamination: Separation of glue joints in plywood or laminated members. Tap the surface lightly; a hollow sound compared with a solid area indicates a void.
• Compression failures: Appear as fine wrinkles running across the grain, usually on the compression face of a bent member. Any compression failure is cause for rejection.
• Glue-joint integrity: Joints must show no separation, discoloration at the bond line, or porosity. AC 43.13-1B lists acceptance criteria for each defect type.

Record every finding on a standardized inspection form with the part identification, the defect type, its location, and your go/no-go decision referenced to the applicable AC paragraph.

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Scarf-Joint Splice Repair

When a spar or stringer has localized damage, a scarf-joint splice restores full load transfer. AC 43.13-1B specifies a minimum slope of 1:10 (rise to run) for softwood members—meaning for every 1 inch of depth, the taper must extend at least 10 inches in length. This distributes shear stress gradually across the adhesive bond line.

Adhesive selection is critical:

• Urea formaldehyde (plastic resin glue): Suitable for interior structures not exposed to sustained moisture. Mix to manufacturer's specified powder-to-water ratio.
• Resorcinol formaldehyde: Water-resistant and used where moisture exposure is anticipated. Produces a dark glue line. Both require clean, freshly machined mating surfaces.

Clamping pressure must match manufacturer data—typically in the range specified on the product data sheet (check and follow it exactly). Pressure that is too low leaves voids; pressure that is too high starves the joint of adhesive. Use calibrated C-clamps or a fixture and maintain pressure throughout the full cure time at the temperature specified by the manufacturer. After cure, measure the finished splice with a steel rule and calipers; final dimensions must be within 1/32 in of the design values.

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Fabric Covering: Heat-Shrink Polyester

Modern aircraft fabric is most commonly heat-shrink polyester (trade names include Ceconite and Poly-Fiber). It replaces the cotton and linen of earlier eras and offers consistent shrink characteristics when heated with an iron set to the temperature specified by the fabric system manufacturer.

Two-bay fabric patch procedure:

1. Clean: Remove all grease, oil, and old coatings from the repair area using an approved solvent.

2. Prime the structure: Apply a coat of fabric cement or primer to all wood and metal contact surfaces per the covering system instructions.

3. Attach the fabric: Cut the patch to overlap at least two inches beyond the damaged area. Bond edges with fabric cement; the overlap must cover at least one rib bay on each side of the damage—hence a two-bay patch.

4. Shrink: Apply a calibrated iron to shrink the fabric evenly. Begin at low temperature and increase as specified. Avoid over-shrinking, which can distort ribs.

5. Dope: Brush on the first coat of nitrate dope (which acts as a sealer and adhesion promoter), then follow with coats of butyrate dope per the system instructions. Allow proper dry time between coats.

6. Check tautness: Use a fabric tensiometer (Seyboth-type or equivalent) to confirm tension is within the acceptable range for the fabric system in use. Compare readings to the manufacturer's published values.

Inspect the finished patch for bubbles, wrinkles, and edge lifting before any additional finish coats.

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Safety Requirements

This lab involves flammable materials and solvent vapors—treat every step as a fire hazard.

• Wear a respirator fitted with organic-vapor cartridges whenever applying dope, solvent, or adhesive. Half-face respirators must be fit-tested.
• Eliminate all ignition sources—no open flames, no sparks—in the doping area. Ensure mechanical ventilation moves air away from the work zone.
• Wear chemical-resistant gloves and safety glasses when handling adhesives and solvents.
• Review the Safety Data Sheet (SDS) for every chemical before use.
• Dispose of dope-soaked rags in a covered metal container; spontaneous combustion is a real hazard.
• Follow your school's lockout/tagout procedures if any powered equipment (sanders, drills) is used to shape the scarf joint.
• Keep a Class B fire extinguisher within reach of the doping station.

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Reference Summary

All acceptance criteria, repair dimensions, adhesive guidance, and fabric procedures referenced in this lesson come from AC 43.13-1B. Always verify that you are using the current revision of the applicable chapters and that any product you select is approved for use with the covering system on the specific aircraft you are maintaining.